This invention relates to a cathode ray tube and more particular, to an antistatic layer and a light filtering layer provided in front of a faceplate of the cathode ray tube.
It is known that a cathode ray tube can reproduce letters and pictures by electron beam bombardment of phosphor screen formed on an inner surface of a faceplate of glass. The electron beam is emitted from an electron gun assembly placed inside a neck of an envelope including the faceplate. The phosphor screen includes dot-shaped or stripe-shaped red, green and blue phosphors which are distributed regularly on the inner surface of the faceplate.
The cathode ray tube has a defect that a contrast of the reproduced images is deteriorated under bright ambient light. In order to improve the contrast, modification for reducing the light transmissivity of the faceplate has been generally employed. For example, it has been proposed that a glass plate (neutral filter), which has an almost uniform transmissivity for light in the visible light region, is fitted on the front surface of the faceplate. It, however, is undesirable for the reproduced images to use the neutral filter, since brightness of the reproduced images is reduced in spite of improvement of the contrast. That is, when the transmissivity of the plate is designated as T, brightness of the reproduced images through the faceplate is reduced proportional to the transmissivity T. On the contrary, ambient light reflected to viewers is reduced proportional to T.sup.2. Thus, the contrast of the reproduced image is improved. However, it is inevitable to reduce the brightness of the reproduced images.
Another cathode ray tube having a faceplate or a glass plate in front of the faceplate containing neodymium oxide (Nd.sub.2 O.sub.3) for improving the contrast without reduction of the image brightness has been proposed in U.S. Pat. No. 4,728,856 and Japanese Patent Disclosure No.57-134848, 57-134849 and 57-134850. Since the faceplate and plate containing Nd.sub.2 O.sub.3 act as a light filter, which has a steep main absorption band at 560 nm .about.615 nm and a secondary absorption band at 490 nm .about.545 nm, because of selective light absorption characteristics of neodymium oxide, the red and blue color purity of the reproduced images are improved and thus the contrast is improved to some extent.
However, a remarkable improvement of the contrast has not been achieved in the cathode ray tube in spite of utilization of selective light absorption characteristics. Namely, when the contrast improvement of the light filter containing neodymium oxide is evaluated by using BCP (Brightness Contrast Performance) as an index, the BCP of the filter is 1.ltoreq.BCP.ltoreq.1.05. It is clear from the value of the BCP that the contrast is not sufficiently improved. The BCP represents the contrast improvement ratio to the contrast improvement in case of using the neutral filter mentioned above as the standard. And the BCP can be also expressed as BCP=.DELTA.B/.sqroot..DELTA.Rf the brightness reduction ratio is designated by .DELTA.B and the reduction ratio of the ambient light reflectivity is designated by .DELTA.Rf.
Also, since the filter containing neodymium oxide has the main absorption band in the wavelength range of 560 nm.about.615 nm and, moreover, the main absorption band has the steep region, of which width is 5 nm.about.10 nm in the wavelength region of 560 nm.about.570 nm, the color of &he glass plate and the faceplate (so called as body color) change due to the ambient light. In particular, the body color becomes red under the ambient light from incandescent lamps. As a result, the parts of the images with low brightness, such as the black color and shadows, take on a reddish tinge, and thus, quality of the images is deteriorated.
Moreover, the cost of the filter increases due to a high cost of neodymium.
The cathode ray tube has another problem due to the glass faceplate. Since the surface resistance of the faceplate is high, static charges due to the electron beam accumulate on the faceplate during tube operation. Because of the accumulation of the static charges, dust and fluff in the atmosphere are absorbed on to the outer surface of the faceplate. Also, when someone touch the faceplate during tube operation, they receive an electrical shock.
In order to solve the problems due to the accumulation of the static charges, it has been proposed that the outer surface of the faceplate is covered with an antistatic layer which can discharge static charges accumulated on the faceplate during tube operation. For example, it is disclosed in U.S. Pat. No.4,563,612 issued on Jan. 7, 1986 that a cathode ray tube has an antistatic, glare-reducing, image-transmitting coating on an external viewing surface of a glass viewing window. The coating has a rough surface for imparting the glare-reducing characteristics and is composed essentially of a silicate material and a metallic compound in proportions to impart the desired antistatic characteristics without substantially degrading the image-transmitting capability of the coating.
Further, it is also disclosed that the formulation may contain pigment particles and/or dyes to reduce the brightness up to about 50 percent of its initial value and/or to modify the spectral distribution of the transmitted image.
However, the coating can not exhibit a satisfactory antistatic effect in practical use. Namely, since the silicate material composing the coating substantially has no conductivity, resistance value of the coating is not sufficiently reduced even if the small amount of metal compounds are contained in the coating. Further, when the amount of the compound added is increased to reduce the resistance value, strength and optical characteristics of the coating are deteriorated.
Another cathode ray tube for solving the accumulation of the static charges is disclosed in Japanese Patent Disclosure No. 61-118946. An outer surface of a faceplate is covered with double layers, which consists of an antireflection layer and an antistatic layer formed on the antireflection layer. The antireflection layer consists of transparent SiO.sub.2 and has rough surface for improving the contrast of the reproduced images. The antistatic layer is formed on the outer surface of the faceplate by spraying a solution which contains an alcoholate of silicon as its main constituent and contains silanole radical.
Since the antistatic layer can absorb moisture in the atmosphere due to the silanole radical, the resistance value of the layer can be effectively reduced. However, when using the antistatic layer, the silanol radical is reduced with the passage of time through the progressive glassification of the silicon forming the basis of the layer. Because of reduction of the silanol radical, the resistance value of the layer increases in accordance with reduction of the moisture absorption capability. As a result, antistatic effect is deteriorated. Accordingly, the antistatic layer lacks stability of antistatic characteristics.